(1) Field of the Invention
The present invention relates to a vessel comprising a composition having an improved stability, which comprises an ethylene/vinyl alcohol copolymer and a propylene resin or a thermoplastic resin having in the main chain or side chain a carbonyl group derived from a carboxylic acid, carboxylic acid salt, carboxylic anhydride, carboxylic acid amide, carboxylic acid ester or carbonic acid ester. More particularly, the present invention relates to an improvement by which the above-mentioned resin composition is stabilized against gelation, discoloration and reduction of gas barrier properties caused when the resin composition is heat-molded into a vessel. Moreover, the present invention relates to a single-layer or multiple-layer plastic vessel having at least one layer composed of the above-mentioned resin composition.
(2) Description of the Prior Art
Among various resins, an ethylene/vinyl alcohol copolymer is one of resins most excellent in the resistance against permeation of gases such as oxygen, and this resin is widely used in packaging vessels such as bottles and cups and films where this characteristic is utilized.
This ethylene/vinyl alcohol copolymer is sensitive to the moisture, and the resin is defective in that under such a high moisture condition as a relative humidity of 100%, the oxygen permeation coefficient is increased about 10 times. As means for eliminating this defect, there have been widely adopted a method in which the ethylene/vinyl alcohol copolymer is blended with a lowly water-absorbing resin excellent in the resistance against permeation of steam or water vapor, such as polyethylene or polypropylene, and a method in which a gas barrier layer comprising this copolymer is sandwiched between inner and outer surface layers of the above-mentioned lowly water-absorbing resin to form a laminate structure in which the influence of the moisture on the ethylene/vinyl alcohol copolymer is reduced.
In Japanese Patent Publication No. 30104/76, it is taught that when a two-component or three-component resin composition comprising an ethylene/vinyl alcohol copolymer, a polyolefin and optionally a thermoplastic resin having a carboxylic acid group or its derivative group is extrusion-molded under certain conditions, there can be obtained a molded structure having a layer in which the ethylene/vinyl alcohol copolymer or polyolefin is preferentially distributed.
Furthermore, in Japanese Patent Publication No. 11263/77, it is taught that when an ethylene/vinyl alcohol copolymer and a polyolefin are co-extruded, if a thermoplastic resin having a carboxylic acid group or its derivative group is incorporated in one or both of the two resin layers, the interlaminar adhesiveness between the two resin layers can be improved.
A multi-layer plastic vessel comprising a polypropylene layer excellent in the moisture resistance and mechanical characteristics and an ethylene/vinyl alcohol copolymer layer excellent in the oxygen barrier property, which are laminated through an adhesive lyer composed of a maleic anhydride-modified polyolefin, is used in the fields of cosmetics, chemicals and toiletry products as a vessel excellent in the content-preservative property in the form of a bottle, a cup, a pouch or the like.
Furthermore, in forming a bottle or cup, scraps of the laminate or layer are inevitably formed, and these scraps are used as the regrind and reclaimed as constituents of the co-extrudate laminate.
However, if a blend comprising an ethylene/vinyl alcohol copolymer, a thermoplastic resin having a carboxylic acid group or its derivative group and optionally a polypropylene is placed under melt-molding conditions, deterioration such as gelation, discoloration (scorching) or degradation of the gas barrier property is caused. From the results of research made by us, it was confirmed that the deterioration of this resin composition is due to deterioration of the ethylene/vinyl alcohol copolymer, and no effective means for preventing this deterioration has been developed.